Pre-reg June 2011 - Calculation Revision Section (Leading upto the exam)

[Nik]

Ah yes, seen where I went wrong - still 19/20 in the exam wouldn't be too bad.

[Nik]

Go on then montblanc here's one for you -

A woman with eclampsia has been prescribed an IV infusion of magnesium sulphate. Below is an extract from her prescription:

Magnesium sulphate 20% in 250ml sodium chloride 0.9%

4g magnesium sulphate to be given over 10 minutes, followed by 1g every hour.

A nurse asks you to check her calculations. Which of the following is/are correct ?

1. To give 4g magnesium over 10 minutes, the infusion rate should be set at 120ml/hr.
2. To give 1g magnesium every hour, the infusion rate should be set at 30ml/hr.
3. The first 250ml infusion bag should last for a maximum of 4 hours 30 minutes.

[montblanc]

is the answer b
120+200 is 320 40% of this is 128
had 200g of 4% so 8 grams to begin with plus the 120g added is 128g
the total weight ( from adding the 200g is 320)
therefore have 128 in 320 which equates to 40%
hmmmmm

Well done! The answer is indeed B.

This type of question is regarded as a incorporation (increasing the concentration of the active ingredient)

This is the correct (long winded method)

1. initial weight of drug + added weight of drug X
------------------------------------------------ = ----
initial weight of mixture + added weight of drug 100

2. Rearrange equation to find unknown:

8 + X 40 1
------- = ----- = -----
200 + X 100 2.5

3. 2.5 (8 + X) = 1(200 + X)
20 + 2.5X = 200 + X
1.5X = 180
X = 120g

I can solve this question in 30 seconds, but i use my own method which i can only understand Can put it if you want

[montblanc]

Go on then montblanc here's one for you -

A woman with eclampsia has been prescribed an IV infusion of magnesium sulphate. Below is an extract from her prescription:

Magnesium sulphate 20% in 250ml sodium chloride 0.9%

4g magnesium sulphate to be given over 10 minutes, followed by 1g every hour.

A nurse asks you to check her calculations. Which of the following is/are correct ?

1. To give 4g magnesium over 10 minutes, the infusion rate should be set at 120ml/hr.
2. To give 1g magnesium every hour, the infusion rate should be set at 30ml/hr.
3. The first 250ml infusion bag should last for a maximum of 4 hours 30 minutes.

Answer: 4g in 20ml per 10min, so 120ml per hr - TRUE
1g in 5ml per 1hour, 5ml/hr - False
First hour is 120ml
second hour is 5ml
third hour is 5ml
fourth hour is 5ml
4 1/2 hours is 2.5ml
total is 137.5ml - False

So answer is A only

[montblanc]

You have a 200g ointment containing 4%w/w Salicyclic acid. How much salicylic acid powder do you need to add to make a 40%w/w ointment?

A. 72g
B. 120g
C. 800g
D. 10g
E. 180g

Anyway here is my method:

8g 40
---- = ----
200g 100

cross multiply: 8g x 100 = 800g
200g x 40 = 8000

8000 - 800g = 7200g

100 - 40 = 60g

7200g/60g = 120g

easy peasy

[johnep]

if you add 72grm to 200grm you will have 272grms and will not be exactly 40%.
It is all too much for my poor old enfeebled mind.
johnep

[jaddimom]

Anyway here is my method:

8g 40
---- = ----
200g 100

cross multiply: 8g x 100 = 800g
200g x 40 = 8000

8000 - 800g = 7200g

100 - 40 = 60g

7200g/60g = 120g

easy peasy

i got 75 using alligation :-s

[montblanc]

i got 75 using alligation :-s

you cant use alligation method to solve this equation.

[amersidd]

am having a tug every hr to relese some pressure. it helps do the kinetics questions!